Occupant support with a migration sensitive bladder and method for migration detection

ABSTRACT

A method for detecting occupant position change on a support surface includes establishing a rate of change of pressure in a head end test zone and in a foot end test zone, comparing the head end rate to at least one head end rate limit, comparing the foot end rate to at least one foot end rate limit, and inferring, in response to the comparing steps, whether or not occupant migration has occurred. An associated occupant support includes a frame and a mattress. The mattress includes head and foot zones each having at least one pressurizable bladder. The support also includes a controller adapted to compare a head end rate of pressure change to a head end rate limit, to compare a foot end rate of pressure change to a foot end rate limit, and to infer, in response to the comparing steps, whether or not occupant migration has occurred.

TECHNICAL FIELD

The subject matter described herein relates to an apparatus and methodfor detecting a position or a change in position of an occupant of anoccupant support, such as a hospital bed.

BACKGROUND

Patients in hospitals, other health care facilities and home caresettings may be confined to a bed for an extended period of time. Suchbeds may include articulable components so that the occupant or acaregiver can adjust the profile of the bed. The beds may also includecomponents for applying selected therapies to the occupant. The use ofarticulating and therapeutic features may cause the occupant to migratefrom an optimum location or position on the bed to another location orposition, usually closer to the foot end of the bed. Even ifarticulating and therapeutic features are unavailable or unused, theoccupant can nevertheless migrate from the optimal location to anonoptimal location. Occupant migration refers to, for example, aninvoluntary change in occupant position in at least the longitudinaldirection, and is distinct from intentional movements such as anoccupant voluntarily repositioning himself or being repositioned by acaregiver. The migration, in addition to placing the occupant in anonoptimal location, can also impose undesirable shear stresses on theoccupant's skin. It is, therefore, desirable to monitor occupantlocation so that a caregiver or automated system can assess the need fora corrective or mitigating action.

SUMMARY

The present application describes a method for detecting occupantposition change on a support surface. The method includes establishing arate of change of pressure in a head end test zone and in a foot endtest zone, comparing the head end rate to at least one head end ratelimit, comparing the foot end rate to at least one foot end rate limit,and inferring, in response to the comparing steps, whether or notoccupant migration has occurred. An associated occupant support includesa frame and a mattress. The mattress includes head and foot zones eachhaving at least one pressurizable bladder. The support also includes acontroller adapted to compare a head end rate of pressure change to ahead end rate limit, to compare a foot end rate of pressure change to afoot end rate limit, and to infer, in response to the comparing steps,whether or not occupant migration has occurred.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of the various embodiments of theoccupant support and method described herein will become more apparentfrom the following detailed description and the accompanying drawings inwhich:

FIG. 1 is a schematic side elevation view of a hospital bed having aframe and a mattress in which the mattress is comprised of a head endbladder in a head zone, a foot end bladder in a foot zone and a seriesof intermediate bladders in an intermediate zone between the head andfoot zones.

FIG. 2 is a schematic side elevation view showing a mattress similar tothat of FIG. 1 and also showing fluid and communication lines extendingbetween the bladders and other components.

FIG. 3 is a view similar to FIG. 2 but having multiple bladders in thehead and foot zones.

FIGS. 4-6 are views similar to FIGS. 2-3 in which the intermediate zoneis a foam material that extends into the head and foot zones and inwhich bladders also occupy the head and foot zones.

FIG. 7 is a block diagram showing a generic version of a method ofdetecting a change in occupant location on the mattresses of FIGS. 1-6.

FIGS. 8A and 8B show a data queue and a diagram of a pressure stabilitytest that uses the data from the data queue.

FIGS. 9-10 are a block diagrams showing more detailed versions of thegeneral method of FIG. 7.

FIGS. 11A-11C are a sequence of views showing a bed occupant migratingfootwardly on a bed having multiple bladders in both the head and footzones; FIG. 11D is a graph of bladder pressure versus time illustratingthe method used in conjunction with the bed having multiple bladders inboth the head and foot zones.

FIGS. 12-14 are a block diagram and pressure vs. time graphs for anothermethod of detecting a change in occupant location.

FIG. 15 is a table showing an example of application of the method ofFIGS. 12-14 to a bed having multiple bladders in the head and footzones.

FIG. 16 is a block diagram illustrating a validation test.

DETAILED DESCRIPTION

FIG. 1 illustrates a bed 20 which extends longitudinally from a head end22 to a foot end 24 and laterally from a right side, visible in theillustration, to a left side. The bed comprises a frame 30 whichincludes a base frame 32, an elevatable frame 34, and a deck 36. A liftsystem, represented by links 40 in the illustration, connects theelevatable frame to the base frame so that the elevation of theelevatable frame can be adjusted. Deck 36 comprises an upper body ortorso section 42, a seat section 44, a thigh section 46 and a calfsection 48 corresponding approximately to an occupant's torso, buttocks,thighs and calves respectively. The angular orientations of the torso,thigh and calf sections are adjustable as indicated by angles α, β, andθ in the illustration. Siderails, not illustrated, border the left andright sides of the bed and include a hip indicator which corresponds tohip plane 50 in the illustration. The hip indicator indicates theapproximate longitudinal location at which the occupant's hip should bepositioned so that the occupant is optimally located on the bed.

The bed also includes a mattress 60 supported by the frame. The mattressincludes a head zone 62 having at least one fluid pressurizable bladder64 and a foot zone 66 also having at least one fluid pressurizablebladder 68. It should be appreciated that the head zone and foot zoneare so named because they are located at or near the head end and footend extremities of the mattress; the zones do not necessarily correspondexactly to the location of an occupant's head or feet. For reasons thatwill become apparent the head and foot zones are also referred to hereinas head and foot test zones and the bladders in those zones may bereferred to as head and foot test bladders. The head zone and the footzone are longitudinally separated from each other by an intermediatezone 80.

FIGS. 2-6 show a selection of mattress architectures. FIGS. 2 and 3,like FIG. 1, show a mattress in which the intermediate zone 80 comprisesa series of longitudinally distributed pressurizable bladders 82 whichare the principal medium or structure for supporting an occupant of themattress (i.e. an occupant of the bed) in the intermediate zone. Howevera structure other than bladders, for example a foam, could be used inthe intermediate zone in which case the alternative medium will be theprincipal medium or structure for supporting an occupant in theintermediate zone. The intermediate bladders may be grouped together inone or more groups with bladders of a given group connected to theirneighboring bladders in the same group buy a fluid passage 84. Theintermediate zone of FIG. 2 includes two groups of bladders, a firstgroup comprising three bladders and a second group comprising fivebladders. The intermediate zone of FIG. 3 includes a single group. Apressure sense tube 90 extends from the head bladder (or from each ofmultiple head bladders) from the foot bladder (or from each of multiplefoot bladders) and from one bladder in each group of intermediatebladders to a pressure transducer 92 dedicated to each bladder or group.In bed architectures that employ multiple head and/or foot bladders,such as the architecture of FIG. 3, the bladders in the head zone arepneumatically isolated from each other and the bladders in the foot zoneare pneumatically isolated from each other and each of the bladders hasa sense line 90 and a pressure transducer 92.

FIGS. 4-6 show variations of a third architecture in which a componentother than bladders, such as foam, comprises the principal supportingstructure of the mattress in the intermediate zone and in which the foammedium extends into the head zone, the foot zone or both (asillustrated) for at least partially supporting the occupant. The foammedium has a frame side 96 and an occupant side 98. The head zone andfoot zone each contain at least one bladder. The head zone bladder orbladders may be embedded in the foam medium (FIG. 4) exposed on theoccupant side of the medium (FIG. 5) or exposed on the frame side of themedium (FIG. 6). The architecture of FIGS. 4-6 also include pressuresense lines and transducers as already described, however thesecomponents are not shown in FIGS. 4-6.

A blower 100 is connected to selected bladders by a network of fluidsupply tubes 102 and appropriate valves 104 for supplying fluid,typically ambient air, to the bladders. The blower is connected to eachof the head and foot bladder or bladders 64, 68 and to each group ofintermediate bladders 82 in architectures that employ intermediatebladders (FIGS. 2-3). The blower can be operated to pump air into thebladders thereby pressurizing them. The blower can also be operated tosuction air out of the bladders thereby depressurizing them.Alternatively depressurization can be achieved by venting a bladder toatmosphere.

A controller 110 communicates with blower 100 by way of a communicationlink represented by line 112. Pressure readings from pressuretransducers 92 are communicated to the controller by communication linksrepresented by lines 114. A user interface 116 allows the bed occupant,a caregiver, or other user to communicate instructions to the controllerby way of communication link represented by line 118.

In the above described beds the test bladders, i.e. the head and footend bladders 64, 68, are a set of one or more bladders in the head zonenear the head end of the bed and in the foot zone near the foot end ofthe bed and are separated from each other by a non-test or intermediatezone which may or may not comprise bladders. The quantity of testbladders depends on the degree of granularity considered to beacceptable to carry out the method described below. As seen in FIG. 2,the combined lengths LF, LH of the test zones is greater than mattresslength L reduced by the height H of a minimum height occupant, forexample a 5th percentile female, i.e.LF+LH>L−H  (1)

In the illustrations the lengths of the test zones are shown as equallyapportioned between the head and foot ends of the bed, however unequalapportionment may suffice or even be advantageous.

Alternatively all the bladders on the bed could be employed as testbladders in which case inequality (1) no longer applies.

Controller 110 receives pressure readings from the transducers andoperates the blower and valves to maintain bladder pressure in a rangebounded by upper and lower limits so that bladder pressure remainsapproximately equal to a set point pressure. For example, if anoccupant's weight shifts partly or completely off a bladder, the fluidpressure in the bladder can decrease to or below the lower limit. Inresponse, the controller operates the blower and valves to increase thepressure to the set point pressure. Conversely, if additional occupantweight shifts onto a bladder, the bladder pressure can increase to orabove the upper limit. In response, the controller operates the blowerand valves to decrease the pressure to the set point pressure. Suchregulation of bladder pressure helps ensure that the magnitude anddistribution of the interface pressure imposed on the occupant by themattress remains satisfactory.

FIG. 7 shows a generic version of a method of detecting a change inoccupant location on the mattress. In general, the method conducts astability test on bladder pressures of interest at block 130. Thestability test determines whether or not the pressures of interest areundergoing a transient, and is shown in more detail in FIGS. 8A-8B. Thepressures of interest are the pressure in the head zone bladder orbladders 64 and the pressure in the foot zone bladder or bladders 68.The stability test is carried out because the method of detecting achange in occupant location relies on establishing the existence of achange in pressure per unit time and therefore requires information toensure that the pressures are not changing just before the onset of thepressure change whose rate is to be determined and just after theconclusion of the pressure change whose rate is to be determined,particularly if the pressure change per unit time is based on a simpledifference of two pressures at two different times. Other methods ofdetermining the pressure change per unit time may not require astability test. The example stability test maintains a queue of timevalues t0, t−1 and t−2 and a queue of pressure values P₀, P⁻¹ and P⁻² ateach of those times (FIG. 8A). The queue is periodically updated bydiscarding the least recent time and pressure values and adding a newtime and pressure value to the other end of the queue. The block diagramof FIG. 8B shows that a stability flag is set to TRUE (indicating thatthe pressures are stable) if the absolute values of all three pressuredifferences in the queue are less than a pressure difference thresholdT.

Returning to FIG. 7, if the pressures of interest are stable, the methodproceeds to block 132 where the controller establishes the rate ofchange of bladder pressure in the bladders of the head and foot zones.As discussed below in more detail, certain rates of pressure change mayreveal that an occupant has migrated longitudinally along the bed, or atleast allow an inference that migration has occurred. Determination ofthe rate of change of bladder pressure can be based on observing thetime required to achieve a given change in pressure, or can be based onobserving the pressure change achieved over a fixed time interval, orcan be a calculated Δp/Δt in which neither Δp or Δt are prescribed, orcan be a more sophisticated determination of the time derivative dp/dt.

Regarding rates of change of pressure, an increase in pressure isconsidered to be a positive change in pressure so that the correspondingrate of change is also positive. Expressions of relative rates ofpressure increase (e.g. the actual rate of pressure increase incomparison to a rate limit) are governed by the magnitude of the rate.For example a pressure increase whose rate is 6 pressure units per unittime is less than or slower than a pressure increase whose rate is 8pressure units per unit time and greater than or faster than a pressureincrease whose rate is 4 pressure units per unit time. A decrease inpressure is considered to be a negative change in pressure so that thecorresponding rate of change is also negative. Expressions of relativerates of pressure decrease are governed by the absolute magnitude of therate. For example a pressure decrease whose rate is 7 pressure units perunit time is less than or slower than a pressure decrease whose rate is9 pressure units per unit time and greater than or faster than apressure decrease whose rate is 5 pressure units per unit time.

At block 134 the controller compares the rate of pressure change in thehead zone to at least one head end rate limit, and compares the rate ofpressure change in the foot zone to at least one foot end rate limit,which need not have the same magnitude as the head end rate limit, todetermine if the rates are consistent with an occupant having migratedlongitudinally along the mattress, particularly toward the foot end ofthe bed. A fast rate of change may suggest nothing more than an occupanthaving sat up or otherwise acted in a purposeful way to change hislocation and/or weight distribution on the bed. A slower rate of changemay be consistent with the occupant having migrated along the bed. Avery slow rate may not suggest anything conclusive and therefore, bydefault, would be considered inconsistent with occupant migration. Aninference can then be made, in response to the comparing steps, whetheror not occupant migration has occurred. If the rate comparison suggeststhat the occupant has migrated along the bed the test is considered tobe positive for occupant migration. If the rate comparison suggests thatthe occupant has not migrated along the bed the test is considered to benegative for occupant migration. Typically the rate comparison willcompare the established actual rate of pressure change to an upper ratelimit (suggestive of deliberate occupant movement) and will yield apositive result if the rate being tested is slower than the upper ratelimit. Alternatively the rate comparison will yield a positive result ifthe established rate falls within a given range, i.e. if the establishedrate is slower than an upper rate limit and faster than a lower ratelimit. As an example, considering the single head bladder/single footbladder architecture of FIG. 2, a depressurization of the head bladderthat occurs quickly enough to indicate a rapid rate of occupant movement(for example an intentional movement of the occupant such as sitting up)would not suggest occupant migration, whereas a slower depressurizationcould suggest occupant migration. If it is desired to test theestablished rate against a range rather than against a single bound, therate could also be tested against a lower limit so that adepressurization rate slower than the lower limit also would not suggestoccupant migration. Similarly, a pressurization of the foot bladder thatoccurs quickly enough to indicate a rapid rate of occupant movementwould not suggest occupant migration, whereas a slower pressurizationcould suggest occupant migration. If it is desired to test theestablished foot end rate against a range rather than against a singlelimit, the established rate could also be tested against a lower limitso that pressurization rate slower than the lower limit would notsuggest occupant migration. Yet another example is the combination of adepressurization rate in the head zone and a pressurization rate in thefoot zone, both of which rates are slow enough to suggest occupantmigration or fall within a range suggestive of occupant migration. It isbelieved that the combination test is superior to either of the testsinvolving only the head zone or the foot zone.

Similar tests of the rate of change of bladder pressure in comparison toone or two limits can be carried out with a mattress architecture havingmultiple bladders in the head and/or foot zones, such as those of FIGS.3-6. In such cases the rate test for a given zone (head or foot) will beconsidered positive if it is positive for any bladder in that zone.

FIG. 9 shows a specific variant of the general method. The method beginsat block 140 with an assessment of the stability of the pressures ofinterest as already described. If the pressures of interest are stablethe method proceeds to block 142 where an initial head zone pressurePH_(i) and an initial foot zone pressure PF_(i) are recorded. At block144 the method waits for a period of time equal to t_(w) and thenproceeds to block 146 to reassess the stability of the pressures ofinterest. If the pressures aren't stable the method proceeds to block148 and continues to wait one or more additional time intervals. Whenthe pressures have stabilized the method advances to block 150 where the“final” pressures PH_(f) and PF_(f) are recorded. The method thenproceeds to block 152 where the pressure change rates are established.At block 154A a rate test is conducted. The illustrated rate testcompares the rate RH of pressure change in the head bladder to lower andupper limits RH_(slow) and R_(Hfast). The illustrated rate test alsocompares the rate RF of pressure change in the foot bladder to lower andupper limits RF_(slow) and RF_(fast). The illustrated test is a “weak”test in that it employs a Boolean OR to infer that the occupant hasmigrated along the bed. That is, the occupant is inferred to havemigrated along the bed provided that either the head zone pressure haschanged at a rate bounded by upper and lower limits or the foot zonepressure has changed at a rate bounded by upper and lower limits.

If the outcome of the rate test is TRUE, the inference of occupantmigration can be accepted as conclusive. Alternatively, if the outcomeof the rate test is TRUE the inference of occupant migration may befollowed by a validation step shown at block 156 and described below inmore detail.

FIG. 10 shows a variant of the method similar to that of FIG. 9 exceptthat it uses a Boolean AND to conduct a “strong” test at block 154B sothat the occupant is inferred to have migrated along the bed only ifboth the head zone pressure and the foot zone pressure have changed at arate bounded by upper and lower limits.

FIGS. 11A-11D illustrate the method used in conjunction with a bedhaving multiple bladders in both the head and foot zones. The bedoccupant P is initially located over head bladders B, C, and D and overfoot bladder E (FIG. 11A). At a later time t₁₀ the occupant has migratedfootwardly and is located over head bladders C and D and over footbladders E and F (FIG. 11B). Referring to FIG. 11D, during migration theoccupant's weight shifts off bladder B causing the pressure in bladder Bto decrease to its lower limit, at which time controller 110 commandsoperation of blower 100 and valves 104 to repressurize the bladder toits set point pressure. During migration the occupant's weight alsoshifts onto bladder F causing the pressure in bladder F to increase toits upper limit, at which time controller 110 commands operation ofblower 100 and valves 104 to relieve the pressure in the bladder to itsset point pressure. As discussed previously the rate of pressure decayin head zone bladder B in comparison to a rate limit, and/or the rate ofpressure rise in foot zone bladder F in comparison to a rate limitresults in an inference that the occupant has migrated footwardly. At astill later time t₂₀ the occupant has migrated footwardly a furtheramount and is located over head bladder D and over foot bladders E, F,and G (FIG. 11C). During migration the occupant's weight shifts offbladder C causing the pressure in bladder C to decrease to its lowerlimit, at which time controller 110 commands operation of blower 100 andvalves 104 to repressurize the bladder to its set point pressure. Duringmigration the occupant's weight also shifts onto bladder G causing thepressure in bladder G to increase to its upper limit, at which timecontroller 110 commands operation of a blower 100 and valves 104 torelieve the pressure in the bladder to its set point pressure. The rateof pressure decay in head zone bladder C in comparison to a rate limit,and/or the rate of pressure rise in foot zone bladder G in comparison toa rate limit results in an inference that the occupant has migratedfurther footwardly. Thus, the use of multiple bladders can not onlyallow an inference that the occupant has migrated along the mattress,but can also offer progressively changing information about the actuallocation of the occupant. As already noted the quantity of test bladdersdepends on the degree of granularity considered to be acceptable.

Referring back to FIGS. 9-10, when the method is applied toarchitectures with multiple bladders in the head and/or foot zones (suchas FIGS. 11A-11C) the method would carry out the stability tests (blocks140, 146) recording step (blocks 142, 150) and establishment step (block152) for each of the test bladders in the zone. The rate test step ofblock 154A or 154B would also be carried out for each bladder. Theresult of the rate test would be TRUE if the pressure change rate in anyone of the head zone bladders were less than the upper limit for thatzone. Alternatively the result of the rate test would be TRUE if thepressure change rate in any one of the foot zone bladders were less thanthe upper limit for that zone. In yet another alternative the result ofthe rate test would be TRUE if the pressure change rate in any one ofthe head zone bladders or in any one of the foot zone bladders werewithin upper and lower limits for that zone (i.e. the “weak” testdescribed above). In yet another alternative the result of the rate testwould be TRUE if the pressure change rate in any one of the head zonebladders and in any one of the foot zone bladders were within upper andlower limits for that zone (i.e. the “strong” test described above).

For the architectures that use multiple bladders in the head and seatzones the upper and lower rate limits can be the same from bladder tobladder within each zone, in which case the rate limits for all the headzone bladders may or may not be the same as the rate limits for all thefoot zone bladders. Alternatively, the rate limits could differ frombladder to bladder. Differing rate limits could improve accuracy byaccounting for the weight imposed on the mattress by different parts ofan occupant's body (e.g. head, neck, shoulders/upper back, lower back,buttocks, thighs, popliteal region, calves and heels). Bladder specificlimits could be associated with individual bladders by way of a userinterface 116 and may be based on factors such as occupant height and/ormorphology. Moreover, controller 110 can be programmed to reassociatethe bladder specific limits from one bladder to the next in response toperceived occupant migration.

FIGS. 12-14 show a block diagram and pressure vs. time graphs foranother method of detecting a change in occupant location. As in thepreviously described method, the method of FIGS. 12-14 assumes that thatthe rate of pressure decrease in the head end bladder and the rate ofpressure increase in the foot end bladder are slow enough to beconsistent with occupant migration rather than with a more abrupttransference of weight. The method further assumes that the changes inbladder pressure during occupant migration are fast enough and that thesystem dynamics are slow enough that the actual bladder pressures exceedthe lower limit for the head bladder and exceed the upper limit for thefoot bladder at some time during the occupant migration.

According to the method, as the occupant migrates footwardly on the bed,bladder pressure in the head bladder decreases and bladder pressure ofthe foot bladder increases in the time interval between t0 and t1. Whenthe head end bladder pressure falls below its lower limit and the footend bladder pressure rises above its upper limit the controller commandsoperation of the blower and valves appropriate for restoring bladderpressures to the set point pressures. At time t2 the bladder pressuresbegin to respond. At time t3 the bladder pressures have returned totheir set point values. As the occupant continues to migrate footwardly,his weight is transferred progressively off the head zone and onto thefoot zone. Accordingly, bladder pressures in those zones undergoadditional cycles of change between peak and trough values. At block 170the method counts the number of pressurization and depressurizationcycles that have occurred in the head zone during a time interval Δt. Atblock 172 the method counts the number of pressurization anddepressurization cycles that have occurred in the foot zone during thesame time interval Δt. For the head zone the peak pressure is shown asapproximately the pressure midway between the lower and upper limits,and the trough pressure is a pressure lower than the lower limit. Atblocks 174, 178, 182 a first flag F1 is set if the number of pressurechange cycles per unit time in the head zone exceeds a first countthreshold T1. At blocks 176, 180, 184 a second flag F2 is set if thenumber of pressure change cycles per unit time in the foot zone exceedsa second count threshold T2. Block 190 calculates the average peakpressure P_(PH) in the head zone during the time interval underconsideration. Block 192 calculates the average trough pressure P_(TH)in the head zone during the same time interval. Blocks 194 and 196calculate similar time averaged peak and trough pressures P_(PF), P_(TF)in the foot zone bladders. The method determines ΔPH, the differencebetween the time average peak and trough pressures in the head zone. Atblocks 198, 200, 204 the method determines if that difference of timeaveraged pressures exceeds a pressure difference threshold T3. If so,the method sets a third flag F3. The method also determines ΔPF, thedifference between the time average peak and trough pressures in thefoot zone. At blocks 206, 208, 210 the method determines if thatdifference of time averaged pressures exceeds a pressure differencethreshold T4. If so, the method sets a fourth flag F4. As indicated byAND gate 212 the method tests whether all four flags have been set (i.e.all four flags have a TRUE value). If so, the test produces a TRUEresult thereby inferring that the occupant has migrated longitudinallyalong the bed.

In the example of FIGS. 12-14 the pressure cycles are depicted as havingapproximately equal amplitudes and periods and are shown as being 180degrees out of phase. However in practice the amplitudes and periods maybe unequal and the phase difference, if any, may be other than 180degrees.

If the mattress head zone or foot zone has only a single bladder as inFIG. 2, the above described method would rely on the pressure cyclecounts and the pressure differences associated with the single head andfoot bladder. Referring to FIG. 15 if the mattress head or foot zone hasmultiple bladders (as in FIGS. 3-6) the test for cycle count would beconsidered TRUE for the head zone if that test were TRUE for any one ofthe bladders in the head zone, and the pressure difference test would beconsidered TRUE for the head zone if that test were TRUE for any one ofthe bladders in the head zone. Similarly, the test for cycle count wouldbe considered TRUE for the foot zone if that test were TRUE for any oneof the bladders in the foot zone, and the pressure difference test wouldbe considered TRUE for the foot zone if that test were TRUE for any oneof the bladders in the foot zone. As seen in FIG. 15 the cycle counttest and the pressure difference test need not be TRUE for the samebladder in the head zone. Likewise the cycle count test and the pressuredifference test need not be TRUE for the same bladder in the foot zone.However imposing the criterion that both the cycle count test and thepressure difference test must be TRUE for the same bladder may bebeneficial. Moreover, it may be beneficial to require that the cyclecount test and the pressure difference test yield a TRUE result forcorresponding bladders in the head zone and in the foot zone. A headzone bladder and a foot zone bladder are corresponding bladders if theirrelative responses to occupant migration (e.g. the outcome of the cyclecount test, the pressure difference or some other suitable test) arecorrelatable. One simple example is a foot end bladder that can beexpected to become more heavily loaded and a head end bladder that canbe expected to become more lightly loaded approximately concurrentlywith the head end bladder becoming more heavily loaded as an occupantmigrates longitudinally along the mattress.

In each of the above described methods the inference drawn from thechanges in bladder pressure during an interval of time may be acceptedas conclusive. Alternatively, a validation test can be conducted as seenat block 156 of FIGS. 9-10. One example test is referred to as a dynamicor active test because it involves making an intentional change inbladder pressure. This differs from the tests already described whichcan be considered to be passive tests because they do not involve makingan intentional change in bladder pressure.

FIG. 16 shows one embodiment of an active test in which at least onetest bladder in the head zone and one in the foot zone are pressurizedfrom an initial pressure P_(i) to a specified target pressure P_(t).Block 210 records the initial pressure P_(i) and the time t_(i) at whichthe pressure reading is taken. The pressurization occurs at block 212.At block 214 the method records the time at which the target pressureP_(t) is achieved. At block 216 the elapsed time for pressurization isdetermined. A bladder that is not involved in supporting an occupantwill achieve the target pressure more quickly than a bladder that isinvolved in supporting the occupant. Hence, block 218 compares thepressurization times to standard times required for pressurization, plusor minus a tolerance, which gives an indication of whether the bladderin question is loaded (playing a role in supporting the occupant—block220) or unloaded (not playing a role in supporting the occupant—block222). By comparing the pressurization times to the standardpressurization times that would be expected for a properly positionedoccupant, an assessment can be made of the occupant's position on themattress and whether that position is satisfactory or is consistent witha migrated occupant. If desired the standard pressurization times can becorrelated with particular segments of an occupant's body. For examplean occupant's head and shoulders might apply more weight on the bladdersthan the occupant's neck, and the occupant's shoulders might apply moreweight than that occupant's head. Accordingly, a group of bladders thatexhibits, a moderate pressurization rate, a faster pressurization rate,and a slow pressurization rate slower than the moderate rate can be usedto assess the occupant's position on the mattress.

Other ways of analyzing the dynamics of bladder pressurization are alsocontemplated. For example in a mattress with a multi-bladdered testzone, rather than comparing pressurization times to standard times, onecould compare the relative pressurization times of the bladders to eachother. Bladders exhibiting pressurization times that differ by at leasta threshold amount, would provide information about the occupant'slocation on the mattress, particularly if those bladders are adjacent toeach other in the same zone and/or the identity of the loaded andunloaded bladders in each zone is consistent with a possible occupantposition, satisfactory or otherwise. For example referring again to FIG.11A-11B, assume that the occupant initially occupied four test bladders,three in the head test zone (bladders B, C, D) and one (bladder E) inthe foot test zone (FIG. 11A) and that a passive test resulted in aninference that the occupant had, over time, migrated footwardly by onebladder width W (FIG. 11B). If the active test reveals that the occupantstill occupies four bladders and that the identity of those bladders isbladders C, D in the head test zone and E, F in the foot test zone, thenthe active test validates the inference from the passive test that theoccupant has migrated one bladder width toward the foot end of the bed.

If the test zone comprises multiple bladders the bladders can bepressurized for the active test concurrently, nonconcurrently in groups,or nonconcurrently individually.

An alternative to an active test based on bladder pressurization time isto vent the bladder and determine the time required fordepressurization. Bladders involved in supporting the occupant willdepressurize more quickly than those not involved in supporting theoccupant

If the dynamic test is carried out on the basis of bladderpressurization, the test results may benefit from a preliminary step ofdepressurizing the test bladder prior to pressurizing it. The testresults may also benefit from a preliminary step of depressurizing oneor more bladders on or both sides of the test bladder.

If the dynamic test is carried out on the basis of bladderdepressurization, the test results may benefit from a preliminary stepof overpressurizing the test bladder prior to depressurizing it. Thetest results may also benefit from a preliminary step of depressurizingone or more bladders on either side of the test bladder instead of or inaddition to overpressurizing the test bladder.

If patient migration is determined to have occurred, one or more actionscan be taken. Examples include issuing an alarm, taking an actionintended to mitigate additional migration such as placing the deck in ahead down orientation, and taking an action to mitigate any adverseeffects of the migration such as the shear and tissue stretch mitigationtechnique described in pending U.S. patent application Ser. No.12/704,600 entitled “Method and Apparatus for Relieving Shear Induced byan Occupant Support”, the contents of which are incorporated herein byreference. Another technique that may be required is to elevate thepressure in any bladder that is perceived to have been subjected to ahigher load (i.e. occupant weight) than that particular bladder wouldhave otherwise been expected to bear based on normal occupant weightdistribution.

The foregoing disclosure contemplates that controller 110 will carry outan algorithm corresponding to one of the passive tests by default.Alternatively, the controller could be set up so that operation of thealgorithm is selectable and deselectable by a user input to userinterface 116. In either case, the algorithm's progress is suppressed ifthe bed (frame or mattress) is adjusted in any way that is predicted tocause the bladder pressures to change in such a way as to mimic thechanges associated with occupant migration. In the limit, one mightunconditionally suppress the test for any adjustment to the bed ratherthan try to predict which adjustments might cause a false positive foroccupant migration. Example adjustments include changing the angularorientation of a deck section or inflating a turn assist bladder.Alternatively, one could delay the onset of the commanded adjustmentuntil the test progressed to a point where the adjustment wouldn'taffect the test. However it is believed this may be less satisfactory tothe occupant and/or caregiver, and would also have to exempt timecritical adjustments, such as adjusting the bed to a condition suitablefor CPR.

Although this disclosure refers to specific embodiments, it will beunderstood by those skilled in the art that various changes in form anddetail may be made without departing from the subject matter set forthin the accompanying claims.

We claim:
 1. A method for detecting occupant position change on asupport surface having a head end test zone with at least one head endtest bladder and a foot end test zone with at least one foot end testbladder, the method comprising: passively establishing a rate of changeof fluid pressure in the head end test zone and; passively establishinga rate of change of fluid pressure in the foot end test zone; comparingthe head end pressure rate to at least one head end rate limit;comparing the foot end pressure rate to at least one foot end ratelimit; and inferring, in response to the comparing steps, whether or notoccupant migration has occurred.
 2. The method of claim 1 comprisingpreceding the establishment steps by a pressure stability test.
 3. Themethod of claim 1 in which the at least one head end rate limit is afast head end rate limit, and the at least one foot end rate limit is afast foot end rate limit.
 4. The method of claim 3 in which theinferring step produces a positive result for occupant migration if theestablished head end pressure rate is slower than the fast head end ratelimit or if the established foot end pressure rate is slower than thefast foot end rate limit.
 5. The method of claim 3 in which theinferring step produces a positive result for occupant migration if theestablished head end pressure rate is slower than the fast head end ratelimit and if the established foot end pressure rate is slower than thefast foot end rate limit.
 6. The method of claim 3 including a slow headend rate limit and a slow foot end rate limit.
 7. The method of claim 6in which the inferring step produces a positive result for occupantmigration if the established head end pressure rate is slower than thefast head end rate limit and faster than the slow head end rate limit orif the established foot end pressure rate is slower than the fast footend rate limit and faster than the slow foot end rate limit.
 8. Themethod of claim 6 in which the inferring step produces a positive resultfor occupant migration if the established head end pressure rate isslower than the fast head end rate limit and faster than the slow headend rate limit and if the established foot end pressure rate is slowerthan the fast foot end rate limit and faster than the slow foot end ratelimit.
 9. The method of claim 1 wherein the inferring step produces aresult with regard to occupant migration and the result is accepted asconclusive.
 10. The method of claim 1 wherein if the inferring stepproduces a positive result for occupant migration, the inferring step isfollowed by a validation step.
 11. The method of claim 10 in which thevalidation test includes changing at least one of pressure in the headzone and pressure in the foot zone in response to the inferred positiveresult; determining a head end pressure derivative based on the changein pressure in the head zone and/or determining a foot end pressurederivative based on the change in the pressure in the foot zone; andconducting a comparison of the pressure derivative or derivatives and anassociated pressure derivative threshold; and concluding, in response tothe conducting step, whether or not occupant migration has occurred. 12.The method of claim 11 in which: both the pressure in the head zone andthe pressure in the foot zone are changed in response to the inferredpositive result; the determining step is carried out for both the headend zone and the foot end zone; and the concluding step is positive foroccupant migration if the comparison for the head end zone, thecomparison for the foot end zone, or both indicate occupant migration.13. The method of claim 9 in which a conclusion of occupant migration isfollowed by one or more steps comprising: issuing an alarm; taking anaction intended to mitigate additional migration; and taking an actionto mitigate adverse effects of the migration.
 14. The method of claim 10in which a conclusion of occupant migration is followed by one or moresteps comprising: issuing an alarm; taking an action intended tomitigate additional migration; and taking an action to mitigate adverseeffects of the migration.
 15. The method of claim 11 in which the changein pressure is an increase in pressure; and the concluding step ispositive for occupant migration if at least one of the following occurs:the pressure derivative for the head end zone is faster than thepressure derivative threshold associated with the head end zone; and thepressure derivative for the foot end zone is slower than the pressurederivative threshold associated with the foot end zone.